The present disclosure relates to a scanning optical apparatus and an image forming apparatus incorporated with the scanning optical apparatus.
Image forming apparatuses that employ electrophotography such as printers and copiers are configured to perform scanning with a light beam to thereby form a latent image on a photoconductor drum. The scanning with the light beam is realized by a scanning optical apparatus. The scanning optical apparatus includes a laser diode (LD) serving as the light source, a collimator lens, a cylinder lens, a polygon mirror, and an fθ lens, and deflects with the polygon mirror the light beam from the light source modulated according to the image to be formed, so that the photoconductor drum is scanned with the deflected light beam in a main scanning direction. The polygon mirror includes a plurality of reflecting surfaces that reflect the light beam, for example five surfaces when the polygon mirror has a pentagonal column shape, and a rotary shaft driven to rotate by a driving motor in one direction.
In the scanning optical apparatus thus configured, the respective end portions of the reflecting surfaces adjacent to each other define a predetermined angle according to the number of reflecting surface (108 degrees in the case of pentagonal column shape). When the polygon mirror is made to rotate at a high speed a turbulent flow of air is generated at the corners between the reflecting surfaces, and therefore dust sticks to the leading end portions of the respective reflecting surfaces in the rotating direction owing to the turbulent flow, thus forming fog on the reflecting surfaces. The fog reduces the reflectance of the light beam, thereby degrading the quality of the corresponding portion (marginal portion) of the image to be formed. Here, the light beam reflected at the position where the fog is formed is not only led to the surface to be scanned but also introduced in a beam detect (hereinafter, BD) sensor, and utilized to generate a reference signal that serves as a reference for starting the scanning of the surface to be scanned. Accordingly, the decline in reflectance of the light beam due to the fog disables the generation of the scan start signal, which may lead to malfunction of the apparatus.
As a remedy for the mentioned drawback, a technique of utilizing only a portion of the reflecting surface of the polygon mirror where fog is not assumed to be formed, thereby preventing degradation in image quality, has been developed.
In addition, a scanning optical apparatus without the reflecting mirror and the photodiode (BD sensor) for generating the scan start signal has been developed, for the purpose of reducing the number of parts and simplifying the assembly and adjusting works. This scanning optical apparatus is configured to detect laser beam emitted from a laser oscillation element serving as the light source and reflected by a scanning mirror so as to return to the laser oscillation element, and utilize the detected light to generate the scan start signal. The returned light is detected using random signals generated in the driving current of the laser oscillation element upon receipt of the light.